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@PHDTHESIS{Amrhein:638426,
      author       = {Amrhein, Tim},
      othercontributors = {Weinelt, Martin and Kuch, Wolfgang},
      title        = {{U}ltrafast 4f multiplet excitation in rare-earth metals
                      studied with {X}-rays},
      school       = {FU Berlin},
      type         = {Dissertation},
      reportid     = {PUBDB-2025-04055},
      pages        = {125},
      year         = {2025},
      note         = {Dissertation, FU Berlin, 2025},
      abstract     = {Optically driven fundamental processes initiating ultrafast
                      magnetization dynamics in 4f rare-earth metals have been
                      studied for many years. Attempts were made to explain the
                      various demagnetization timescales observed for different 4f
                      metals, considering e.g. magnon generation or
                      electron-phonon driven spin-flip scattering. Excitations
                      within the 4f electronic system, though, were not involved
                      in the discussion, since they are not directly accessible by
                      optical stimuli. This thesis reports on time-resolved
                      experiments performed at the freeelectron laser facilities
                      EuXFEL and FLASH which identified an up to now disregarded
                      mechanism provoking 4f electronic excitations on ultrafast
                      timescales indirectly via interaction with the laser-excited
                      5d6s valence electrons in 4f metals. By analyzing ultrafast
                      spectral changes in X-ray absorption (XAS) and resonant
                      inelastic X-ray scattering (RIXS) signals from terbium metal
                      with the help of atomistic simulations, inelastic 5d-4f
                      electron-electron scattering is found to transfer energy and
                      angular momentum between the 4f and 5d systems. As a
                      consequence, 4f electrons are excited from the ground state
                      7F6 into energetically higher 7FJ multiplets. Based on these
                      findings, a three-temperature model has been developed,
                      separating temperatures for the 4f electrons, the 5d valence
                      electrons and the lattice. By fitting the simulation based
                      on the three-temperature model to the experimental data, the
                      coupling between 4f and 5d electrons is found to increase
                      with the amount of valence electrons, able to distribute
                      enough energy to lift the 4f electronic system to the
                      energetically lowest excited 4f multiplet 7F5. According to
                      the 3rd rule of Thole and van der Laan, the absorption
                      branching ratio of M5 to M4 decreases with the reduction of
                      the total angular momentum J, as expected for transitions
                      into energetically higher multiplets 7FJ=5,4,..0 of the 4f
                      electronic system. With time-resolved XAS experiments at the
                      FemtoSlicing facility BESSY II, it was demonstrated, that
                      this correlation can be used to probe and evaluate 4f
                      multiplet excitations, even with sources exhibiting limited
                      energy resolution. The reported 5d-4f scattering mechanism
                      transiently alters the magnetocrystalline anisotropy and
                      hence the coupling of the 4f system to the lattice. With up
                      to $22\%$ of all probed atoms being excited to energetically
                      higher multiplets, inelastic 5d-4f scattering is deemed to
                      have a significant impact on the ultrafast demagnetization
                      in terbium metal and must be taken into account for a full
                      description of the magnetization dynamics in rare-earth
                      metals.},
      cin          = {DOOR ; HAS-User},
      cid          = {I:(DE-H253)HAS-User-20120731},
      pnm          = {6G2 - FLASH (DESY) (POF4-6G2)},
      pid          = {G:(DE-HGF)POF4-6G2},
      experiment   = {EXP:(DE-H253)F-PG1-20150101},
      typ          = {PUB:(DE-HGF)11},
      urn          = {urn:nbn:de:kobv:188-refubium-48744-7},
      doi          = {10.3204/PUBDB-2025-04055},
      url          = {https://bib-pubdb1.desy.de/record/638426},
}